Thermostatic shower valve replacement

[RobKillalea]

Hi DIYer here. Trying to replace concealed shower thermostatic valve. Please could you advise on how to remove valve. Very large box spanner? Many thanks

 

[Ben-gee]

Put a large slotted screwdriver in the left hand slot and strike it downwards with a hammer using short sharp blows. You have to have the screwdriver almost vertical, resting on the bottom ledge of the slot.